It is possible for a user to receive a message that they do not have permission to reply to, for example a teacher/admin may have the capability to message a student who does not have the capability to send messages. There is already handling in the backend to prevent the student in this example from sending a message, but there is no indication in the UI until they try to send a message:

This is not a great experience because (a) the message is not very user friendly (using IDs etc), and (b) the timing of the message means the student would have to first write an entire message and hit send to see it, which is a frustrating experience that costs them unnecessary time.

The purpose of this issue is to make it more obvious in the UI before the user tries to send a message. I would propose these requirements:

1. Update the error message string so that it is more user friendly. We can probably make it more generic, for example "You do not have permission to send a message in this conversation".
2. Persistently display the language string at the bottom of conversations the user cannot reply in, using the same elements currently displaying the error after they try to send the message. This should preemptively make it obvious to the user that they will not be able to reply to the message.
3. If possible, disable the messaging textarea and/or send button, so it is really obvious to the user that they cannot send a message.
4. We must retain the existing backend error functionality in addition to the above, particularly for security reasons, so that if someone trying to bypass the UI (eg sending directly via web service) that still receives an error response and the message is not sent.

I believe that the messaging textarea is common across all conversations, and it is possible that the user may be able to message some users and not others, so we will need to investigate whether that situation is possible (eg contacts vs non contacts), and if so, if it is possible to check whether the user is allowed to send in a conversation when that conversation is loaded, and if they don't have the access, disable the relevant form elements (and if they are allowed, make sure we re-enable the the elements so they can send messages in those cases). It also means we should not clear the text area when we disable or enable it, because the text carries between conversations and we don't want them to lose what they've written somewhere else if they flick between conversations.